Check with any radiant equipment supplier. They should be able to supply all the data you would need for a successful installation. Radiant is used in commerical garages a lot, especially in colder climates, so its use in a residental garage should be no problem at all.

EZ to do but how do you rationalize it as green?

My thoughts also? The garage is an enclosed environment, should be about 20 degrees warmer than outside without heat, and its for your car? The equipment, labor and subsequent costs to run would offset your house heating admirably. If you use 2x4 walls for the garage, perhaps spending money on spray foam would be a better return?? No heat needed, and its still warmer that outside!

Green goes beyond which products you use, there is also the usage cost for what could be considered extravagant. Now I understand it's your right to spend your money as you see fit, so it is ultimately your choice!